staging: r8188eu: change last argument type of the usb_write_port function

usb_write_port writes only xmit_buf object data to device.
In addition, an appropriate name for this argument is used.

Signed-off-by: Ivan Safonov <insafonov@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>